The present invention relates generally to a compressor, and more particularly, to a compressor in which distortion of the output signal thereof is minimized.
In a communication system, a compressor is utilized to compress the amplitude of an input signal to be transmitted over a communication or transmission channel, in order to ensure that the amplitude of the signal does not surpass the linear operation range of the transmission channel. The compressor non-linearly compresses the input signal by reducing the degree that relatively small amplitude portions of the signal are compressed and increasing the degree that relatively large amplitude portions of the signal are compressed. The compressor also inherently restricts the extension of the sidebands of the transmitted signal, thereby minimizing cross-talk between adjacent communication channels in both the transmitter and receiver. The communication system also includes an expander, which serves to expand the amplitude of the compressed signal to its original level.
A compandor is an apparatus which includes both a compressor and an expander, and which functions to improve the signal-to-noise (S/N) ratio of a signal transmitted over a noisy transmission channel, by compressing the signal at the transmitter prior to noise exposure, and then expanding the compressed signal back to its original form at the receiver, after noise exposure. The compandor compensates for signal level differences between high volume level sound and low volume level sound, e.g., in telephone systems and the like, wherein high volume level sound is linearly attenuated by the compressor in the transmitter and linearly expanded by the expander in the receiver to its original level.
With reference now to FIG. 1, there can be seen a block diagram of a conventional compressor, which includes a summing amplifier 12, a rectifier 14, a gain controller 16, and an amplitude limiter 18, interconnected as shown.
The summing amplifier 12 compresses an input signal S1 to a predetermined amplitude, to thereby output a compressed signal S2. The rectifier 14 full-wave rectifies the compressed signal S2 to a direct voltage, and then converts the direct voltage into a direct current. The gain controller 16, in response to the output voltage of the summing amplifier 12 and the direct current output of the rectifier 14, exponentially increases or decreases the amplitude of the input signal S1 applied to the input of the summing amplifier 12. The direct current output from the rectifier 14 is proportional to the amplitude of the compressed signal S2.
When the amplitude of the input signal S1 is relatively large, the summing amplifier 12 increases the degree of compression relative to when the amplitude of the input signal S1 is relatively small. The amplitude limiter 18 prevents the compressed signal S2 output from the summing amplifier 12 from being overmodulated, and outputs the signal S3 to a transmitter (not shown) or automatic level controller (not shown).
With reference now to FIGS. 2A and 2B, there can be seen waveforms of the above-described signals produced by the conventional compressor depicted in FIG. 1. More particularly, FIG. 2A illustrates the waveforms of the compressor signals when the amplitude of the input signal S1 is smaller than a first predetermined threshold magnitude, wherein the compressed signal S2 has an amplitude smaller than the saturation or peak level S4 of the amplitude limiter 18. Therefore, the compressed signal S2 is not distorted, and thus, the transmitted signal S3 output by the compressor is also undistorted.
FIG. 2B illustrates the waveforms of the compressor signals when the amplitude of the input signal S1 is greater than the first predetermined threshold magnitude, wherein the compressed signal S2 has an amplitude greater tan S4. Consequently, the amplitude limiter clips the portion of the compressed signal S2 which is greater than S4, thereby distorting the compressed signal S2, and hence, the transmitted signal S3.
In order to eliminate the above-described type of output signal distorted, the compressor is designed to reduce the degree of compression of the input signal S1 when the input signal S1 has a relatively small amplitude, and to increase the degree of compression of the input signal S1 when the input signal S1 has a relatively large amplitude. In this way, as long as the amplitude of the input signal S1 is not greater than a second predetermined threshold magnitude above which the compressed signal S2 is clipped by the amplitude limiter 18 at the level S4, the signal-to-noise (S/N) ratio of the compressed signal S2, and hence, the transmitted signal S3, is enhanced.
However, when the amplitude of the input signal S1 exceeds the second predetermined threshold magnitude, the amplitude of the compressed signal S2 will still exceed the peak level S4 of the amplitude limiter 18, whereby the portion of the compressed signal S2 greater than S4 is clipped by the amplitude limiter 18, thereby distorting both the compressed signal S2 and the transmitted (output) signal S3.
In order to overcome the above-described distortion problem, an automatic level controller (not shown) is added to the conventional compressor to prevent the amplitude limiter 18 from clipping the compressed signal S2 when its amplitude exceeds the second predetermined threshold magnitude. Because the capacitors of the automatic level controller must necessarily be large, they can not be fully integrated with the core circuity of the compressor on a single chip, and thus, must be provided as a peripheral device external to the core circuity of the compressor. Of course, this increases the number of components and manufacturing costs of the conventional compressor.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a compressor which eliminates the above-described drawbacks and shortcomings of presently available compressors. The present invention fulfills this need.